The quantum efficiency of a proportional counter for X-rays is determined by the transmission of the window and the absorption of the detector gas. To achieve a high transmission rather thin windows are used. A thin window has to be supported by a grid to withstand the gas pressure. The X-ray transmission of the support structure is usually energy independent and reduces the transmission by a constant factor (T). Proportional counters with a permanent gas filling have to use metallic window materials like beryllium or aluminum. Metallic window materials limit the detectable X-ray band to energies above 1.5 keV. Detectors using plastic window materials like polypropylene (about 1 |m thick) are able to detect X-ray photons down to 0.1 keV. Because of the gas diffusion through the plastic window such detectors have to use a gas supply system. Figure 2.2 shows the X-ray transmission of a 1 |m polypropylene foil and a 25 |m beryllium foil as a function of energy. The absorption of X-rays in the detector gas, usually a mixture of a noble gas with 5-20% quench gas, depends on the atomic numbers of the gas mixture, the gas pressure, and the dimension of the absorption region. For low energies, the basic constituent of the gas mixture is argon, whereas for higher energies increasing admixtures of xenon or pure xenon with a quench gas is used. The quantum efficiency of the detector can be written:
Energy (keV) Energy (keV)
Fig. 2.2 Left panel shows the transmission of two window materials 25 |im beryllium (Be) and 1 |im polypropylene (PP). The right panel shows the absorption of 1 cm of argon (Ar) and xenon (Xe) at a pressure of 1 bar as a function of X-ray energy 
d and g are the window and gas column densities in g cm 2. jw, jg are the corresponding energy-dependent mass absorption coefficients.
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